The idea that diazepam (Valium) may be involved in carcinogenesis could have extremely serious implications since Valium is the most widely prescribed tranquilizer. Evidence available from preliminary studies carried out in two experimental models - the R3230AC Rat Mammary carcinoma snd the Walker 256 tumor, indicates that diazepam had a dose-related effect on growth of these transplanted tumors. We propose to undertake studies to determine whether diazepam has: 1. Tumor promoting actions on the incidence of spontaneously developing mammary tumors in Balb/c mice, 2. Tumor enhancing actions on the growth of the transplantable R3230AC mammary carcinoma in female Fischer rats. Treatment levels of diazepam will be employed in a dose-range corresponding to therapeutic doses in man. The results of these investigations should provide critical data concerning the role of diazepam in mammary cancer.